When blowing film from thermoplastic material the molten material is extruded through an annular die and the formed tube of material is drawn away from the die and stretched in the direction of the movement. At the same time gas is introduced in the middle of the tube in such an amount that the tube is blown up and expands in the direction perpendicular to the movement. Substantially all the stretching in the two directions takes place before the material has become solidified and after the frost point the tubular body can be flattened and hauled off with the gas remaining in the expanding part of the tube.
The main object of the mentioned stretching in the machine direction (MD) and the transverse direction (TD), respectively, is to decrease the thickness of the film considerably relative to the thickness of the extruded tube of material. However, the stretching also leads to a molecular orientation and this orientation will be permanented in the form of built-in tensions when the material solidifies. These tensions show up for example at heating when the built-in forces are relaxed, and result in a shrinking of the film. Although the molecular orientation can be advantageous in some applications it is, for most purposes, desired that the material has as few built-in tensions as possible. Since it is not possible to avoid tensions entirely, it is desirable to keep them so evenly distributed that the material at the relaxation will shrink essentially uniformly in all directions, which is achieved with materials which at the production are uniformly stretched in both the mentioned directions. However, at blowing it is easier to achieve a high degree of stretching in the machine direction without problems than in the transverse direction. This is a result of the irregularities which are always present in the extruded thermoplastic material, e.g. due to impurities, temperature differences, shear stress from the die or non-uniform cooling. Each irregularity is enlarged with the degree of stretching but the drawing in the machine direction has a certain equalizing effect on the irregularities in this direction but there is no corresponding stabilizing effect in the transverse direction. Problems with the symmetry and stability of the tubular bubble thus limit the ratio of stretching in the transverse direction, and thus also the possible production in a given film blowing equipment.
Although the described problems arise for all thermoplastic materials they are far more troublesome at film blowing from linear thermoplastic materials, such as HD (high density) or LLD (linear low density) polyethylene, than for non-linear materials such as conventional LD (low density) polyethylene. The rheology of a conventional, highly branched LD-polyethylene is such that the tensions in and the viscosity of the material increases with increasing deformation. In general the elongation is concentrated to locally weakened areas in the film so that possible deviations tend to increase unlimitedly until breakage, but for the LD materials this is to a high degree counteracted since the viscosity at the same time increases in areas with a high stretch rate. Since the LD-materials also in other respects have a good cohesion and strength in a molten condition it has for these materials been possible to control the blowing process, also for very large tubular bodies.
The linear thermoplastic materials behave in a quite different manner. The viscosity does not increase, or increases just a little, at stretching with increasing deformation and there are no increases in tension to counteract an ever increasing elongation in the thinner and thinner areas of the film. The tendency of the materials to an explosion-like stretching will, among other things, manifest itself in the shape of the bubble at the production. When the melt has left the die it forms a thin neck, to a certain height, before the material has become sufficiently thin for the gas pressure to overcome the resistance to stretching when the expansion will occur rapidly to the final dimension. The material behaves in the same way where there are local weaknesses in the film, i.e. it stretches with increasing speed. This behaviour, in combination with the generally low melt strength of the materials, result in a difficultly controlled blowing process which does not infrequently give an asymmetrical and unstable bubble and result in final product which is not uniformly stretched. In more serious cases, pinholes are formed in the film and the tube collapses completely. The problems increase with increasing dimensions of the equipment and when attempts are made to increase the production in existing equipment, since the weight of the molten material that has to be stabilized increases and since the film thickness will be relatively smaller so that the problems with instability grow.
Various attempts have been made to solve these problems, e.g. by supporting the film in different manners before the solidification, by modifying the plastic material and making it less linear or by different ways of differentiated and controlled cooling of the tube. Such methods have made it possible to increase the stability in the procuction process but it has not been possible to increase the dimensions of the equipment or the production capacity to any appreciable extent.
The insufficient understanding of the influence of changes in the stretching on the properties of the final material is a further problem. While it is fairly easy to predict the influence of changes in the stretch ratio on the final product when LD-materials are blown, it has been found considerably more difficult to predict the shrink properties of the final products when changes are made in stretching processes for linear materials. This lack of understanding of the basic principles in the stretch process has made it more difficult to solve the above described problems. While it today is fairly easy to blow LD-polyethylene with a high capacity to a diameter of more than 4 meters, equipment for linear polyethylene has far from that capacity. These capacity problems have seriously limited the distribution of linear plastic materials as film material, despite excellent properties otherwise, e.g. a generally lower degree of orientation than for the LD-materials.